A quantum approach to paramagnetic resonance

Andrei V. Butashin, S. O. Demokritov, Natalia M. Kreines +1 more

Nikolai N Rozanov

Patrice Bertet

Almas F. Sadreev

The dynamics of a paramagnetic system (S = 1/2), thermodynamically equilibrium-linked with the radiation field of a resonator is considered by the methods of the quantum theory of fields. The spectrum of the resonance frequencies of the system is found for two relations of the size of the paramagnetic sample L and the wavelengths of the radiation field k−1. It is shown that when L ≪ k−1 Fain’s result, Eq. (1), is valid for the intrinsic line width, if the Zeeman frequency ω0 ≫ck. With L ≳ k−1, (the case of a crystal resonator), and kT>ω0>ρSμB2 (ρS is the paramagnetic ion density in the crystal), the temperature behavior of the intrinsic width is described by formula (33). It is shown that the critical temperature Tc is the temperature of the Bose-Einstein condensation of photons of the resonator radiation field, which is determined only by the paramagnetic ion density in the sample. Estimates are made of the intrinsic broadening of the Zeeman line in certain paramagnetic crystal resonators with impurities of rare-earth ions.

Colin J. Stephen, Anton Tcholakov, Maik Icker +7 more

Electron paramagnetic resonance (EPR) is a valuable tool for physics, chemistry, biology and medicine, providing complementary spectroscopic information to NMR. It has long been known that EPR at high magnetic fields offers greater spectral resolution, but limitations in the THz instrumentation have prevented the full realization of these opportunities. Here we describe an EPR spectrometer at the high magnetic field of 14 T using 396 GHz excitation, which adapts techniques from liquid-state NMR to obtain sharp EPR resonances with a width of 210 ppb (full-width half-maximum). We use this to measure resonance positions, and hence g-factors, with a precision that reaches $\pm$16 ppb. Our use of in-situ liquid-state NMR of our solvent within the same sample improves the accuracy of these measurements: it allows us to reference our EPR measurement back to dilute gas 3He NMR for which quantum calculations are accurate. We measure the g-factor of N@C60 in deuterated toluene as g = 2.002 099 09 (3), where the 3 in brackets means that the uncertainty on the last digit is $\pm$3.

V.A. Dzhrbashyan